The role of juvenile hormone (JH) in insect growth and development is now fairly well understood, and the pure Cecropia hormone and many mimics are now available. Yet the mode of action of juvenile hormone on the cellular and molecular level remains to be explored. Thus, the proposed research is designed to continue on-going work in my laboratory on four specific systems which appear to be ones in which these mechanisms can be explored: (1) The blockage of embryonic development at the blastoderm stage by JH application to the female during the terminal phases of oogenesis. (2) The action JH in promoting the differentiation of Manduca larval crochet epidermis in vitro. (3) The coloration of Manduca larval epidermis by a JH-dependent uptake of a blue protein pigment from the blood. (4) The switchover of the Manduca epidermal cell from a commitment to make larval cuticle to one to make pupal cuticle in the absence of JH at the time of ecdysone action. For (1) an investigation of RNA synthesis in the early embryos and a cytological study of JH-sensitive stage of the terminal phase of oogenesis is proposed. An autoradiographic and biochemical study of the cellular events (DNA, RNA, and protein synthesis) occurring at the time of JH sensitivity is proposed for the three epidermal systems once the propoer conditions are found for JH action in vitro (in (2) and (3)).